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BQ20Z45-R1: What should the OCV be on the output if the device is in shutdown mode?

Mike Mrazik
Expert 1255 points
Part Number: BQ20Z45-R1
Other Parts Discussed in Thread: BQ20Z45, BQ40Z50

Hello,


If FETs are off and are in shutdown mode, what would the output measure from Pack + to Pack - ?  


Thanks,

Mike

  • 0
    TI__Mastermind 42535 points
    The gauge would not see any voltage if the charger is not connected. If the voltage of the charger is less than the charger present threshold, the gauge will remain in shutdown and the pins will see the charger voltage across the pins.
  • 0 in reply to Batt
    Expert 1255 points

    Batt,

    I apologize, I may have been confusing.  There is no charger attached at all, so we are talking about the open circuit voltage on the output of the pack after it shuts off.  Pack - stays at the same level as Batt -, as it is not broken and only has the sense resistor between them.  Pack + is separated from Batt + by the FETs, so in shutdown mode, what is the voltage I should expect to measure across Pack + and Pack -?

    Thanks,
    Mike

  • 0 in reply to Mike Mrazik
    TI__Mastermind 42535 points
    Mike,

    If the fets are off, it will be 0 as the path to the pack pins is broken on the high side.
  • 0 in reply to Batt
    Expert 1255 points

    Batt,

    I don't know if that is true, and in fact have never seen that.  I've typically seen around 0.5 - 0.8V when the FETs are OFF.  In my mind, I will read whatever the pack + side floats to in reference to pack -.  I know for my circuitry that when Pack+ is disconnected from Batt+, I don't have a single resistor that would pull that net down to GND to give me 0V.  Therefore, I'm basically asking what is internal to your part that would pull this net down where you think I should have 0V across Pack+ and Pack- during shutdown?

    I'm not concerned about anything, even if I had 1V on the output during shutdown, the second any sort of load goes on there it drops to 0V.  I put a 100K resistor on and it dropped right down, I'm just looking for what is expected so we can tune our factory's "pass/fail" criteria.

    Mike

  • 0 in reply to Mike Mrazik
    TI__Mastermind 42535 points

    Oh, OK. Now, I understand. If you are letting it float, then yes, it will show whatever internal voltage PACK+ sees wrt PACK-. I thought you were asking me when the gauge is in system (with the load resistance) then if you turn the FETs off, what voltage you would see. The answer to that was 0.

    But yes, you are right, if there is no load or connection between PACK+ and PACK-, it will internally float as you say.

  • 0 in reply to Batt
    Expert 1255 points
    Batt,

    Do you have any idea where this might float internally to your component so I can give my manufacturer some values to expect (example: based on the internal circuitry of the BQ, this will never float above 1V).

    Mike
  • 0 in reply to Mike Mrazik
    TI__Mastermind 42535 points
    I will check with the hw designers and get back to you. Please allow me a day or two.
  • 0 in reply to Batt
    TI__Mastermind 42535 points
    Mike,

    I checked with the hw designers. The bq20z45 is an older core in which PACK+ is not tied internally to GND. Therefore depending on the leakage from the external components, it will leak current until leakage current is stable. This means the voltage would be dependent on leakage.
    In the newer devices like the bq40z50, the PACK is internally pulled down to 0.